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RFID Resonance and addition of a capacitor element

Posted 11 mai 2021, 07:35 UTC−4 Low-Frequency Electromagnetics, RF & Microwave Engineering, Geometry Version 5.6 9 Replies

Kelvin Okenye
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Hallo, I am new to COMSOL simulations in the RF section. I am designing a spiral RFID antenna which should have a resonance frequency of 13.56MHz. I have a coil with an inductance of 2.96uH (which i verified using the ac/dc physics and calculation) and I place a parallel capacitor using a lumped element boundary condition. The spiral has 8 turns and is made of stainless steel having an impedance boundary condition and the boundary feed is a lumped boundary with a feed of 3.3V. However, my model returns a resonance at about 800MHz. Does anyone have an insight of what I am doing wrong? My comsol file is attached.

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9 Replies Last Post 17 mai 2021, 06:20 UTC−4
Edgar J. Kaiser Certified Consultant
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Posted: 4 years ago 11 mai 2021, 08:47 UTC−4

The lumped element is an inductor not a capacitor.

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
The lumped element is an inductor not a capacitor.
Kelvin Okenye
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Posted: 4 years ago 11 mai 2021, 10:04 UTC−4

The lumped element is an inductor not a capacitor.

Hi Edgar, so my coil produces the 2.96uH capacitance and the lumped element is a capacitor(I might have mistakenly changed it when I was deleting the mesh). However, if you set it to a capacitor of about 47pF you still do not get resonance at 13.56MHz but at around 800MHz. Something is still wrong?

>The lumped element is an inductor not a capacitor. Hi Edgar, so my coil produces the 2.96uH capacitance and the lumped element is a capacitor(I might have mistakenly changed it when I was deleting the mesh). However, if you set it to a capacitor of about 47pF you still do not get resonance at 13.56MHz but at around 800MHz. Something is still wrong?
Edgar J. Kaiser Certified Consultant
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Posted: 4 years ago 11 mai 2021, 10:47 UTC−4

You should post your model together with the mesh sequence and the study and solver sequence you used. You don't need to delete the mesh sequence, only delete the mesh. Same with study and solver and dataset. You don't need to delete them, only delete the solution.

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
You should post your model together with the mesh sequence and the study and solver sequence you used. You don't need to delete the mesh sequence, only delete the mesh. Same with study and solver and dataset. You don't need to delete them, only delete the solution.
Kelvin Okenye
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Posted: 4 years ago 11 mai 2021, 12:29 UTC−4

You should post your model together with the mesh sequence and the study and solver sequence you used. You don't need to delete the mesh sequence, only delete the mesh. Same with study and solver and dataset. You don't need to delete them, only delete the solution.

Hi Mr. Kaiser, here is the file once again with the mesh and study.

>You should post your model together with the mesh sequence and the study and solver sequence you used. >You don't need to delete the mesh sequence, only delete the mesh. Same with study and solver and dataset. You don't need to delete them, only delete the solution. Hi Mr. Kaiser, here is the file once again with the mesh and study.

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Kelvin Okenye
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Posted: 4 years ago 11 mai 2021, 16:33 UTC−4

So I did a long sweep from 10MHz to 1GHz to get the resonance frequency and that is how I managed to get a resonant frequency at 800MHz. However, I need my lumped element capacitance to work ?

So I did a long sweep from 10MHz to 1GHz to get the resonance frequency and that is how I managed to get a resonant frequency at 800MHz. However, I need my lumped element capacitance to work ?
Edgar J. Kaiser Certified Consultant
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Posted: 4 years ago 12 mai 2021, 05:17 UTC−4
Updated: 4 years ago 12 mai 2021, 05:19 UTC−4

Kelvin,

you need to remove the coil domain from the emw physics, otherwise the impedance bc doesn't work. You also don't need to mesh the coil domain, only its boundary.

And you won't be even close to 50 Ohm impedance at the port in resonance. I think this kind of RFID antenna is typicall driven in series resonance?

Cheers Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Kelvin, you need to remove the coil domain from the emw physics, otherwise the impedance bc doesn't work. You also don't need to mesh the coil domain, only its boundary. And you won't be even close to 50 Ohm impedance at the port in resonance. I think this kind of RFID antenna is typicall driven in series resonance? Cheers Edgar
Kelvin Okenye
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Posted: 4 years ago 14 mai 2021, 06:43 UTC−4

Kelvin,

you need to remove the coil domain from the emw physics, otherwise the impedance bc doesn't work. You also don't need to mesh the coil domain, only its boundary.

And you won't be even close to 50 Ohm impedance at the port in resonance. I think this kind of RFID antenna is typicall driven in series resonance?

Cheers Edgar

Hey Edgar, So I followed your mesh and boundary advice. However I still do not get resonance. You mentioned driving the antenna in series resonance. Does this mean that the capacitor lumped element is wrongly placed?

>Kelvin, > >you need to remove the coil domain from the emw physics, otherwise the impedance bc doesn't work. You also don't need to mesh the coil domain, only its boundary. > >And you won't be even close to 50 Ohm impedance at the port in resonance. I think this kind of RFID antenna is typicall driven in series resonance? > >Cheers >Edgar Hey Edgar, So I followed your mesh and boundary advice. However I still do not get resonance. You mentioned driving the antenna in series resonance. Does this mean that the capacitor lumped element is wrongly placed?
Paul Kaul
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Posted: 4 years ago 17 mai 2021, 04:46 UTC−4
Updated: 3 years ago 2 juil. 2021, 16:51 UTC−4

Hi...in my case I have not checked the maths yet it won't work at any rate. The harmony between the L and C isn't right. At that recurrence it will be hard to get a capacitor of that size that doesn't act like an inductor. The capacitor ought to be in the scope of 100pF to 1nF and the inductors should then be made to suite.

order pcb

Hi...in my case I have not checked the maths yet it won't work at any rate. The harmony between the L and C isn't right. At that recurrence it will be hard to get a capacitor of that size that doesn't act like an inductor. The capacitor ought to be in the scope of 100pF to 1nF and the inductors should then be made to suite. [order pcb](https://www.7pcb.com/Order-PCB.php)
Edgar J. Kaiser Certified Consultant
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Posted: 4 years ago 17 mai 2021, 06:20 UTC−4

Kelvin,

you built a parallel LC circuit that shows high impedance at resonance. In a real circuit you typically prefer low impedance, e.g. for a transistor driver. In this case you would have the coil and the capacitor in a series circuit.

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Kelvin, you built a parallel LC circuit that shows high impedance at resonance. In a real circuit you typically prefer low impedance, e.g. for a transistor driver. In this case you would have the coil and the capacitor in a series circuit.

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